Well this is my instructable for my coaxial airgun that I put together.
It costs about $15 to construct, and is very fun to mess around with.
It shoots at roughly 600fps+ at 120psi with a .12gram airsoft bb and the excact specifications of the airgun made in this instructable.

And I must state that ***IT IS NOT MY FAULT IF YOU OR OTHERS ARE HURT OR KILLED AS A RESULT OF THIS AIRGUN***

And a couple tips (pretty much follow them or you will die,no joke)
*Never pressurize it over 140psi, even though it says rated to 480psi on the side.
*Never look down the barrel, even when you dont think it is loaded or pressurized
*Never aim at anything you do not intend to destroy
*Do not use this in an airsoft war, this is meant to just be something to mess around with. It will penetrated human flesh.
*Give the pvc cement and epoxy a full 24 hours to dry, no less.
*Have fun

Make sure you understand all those tips before going to next page.

And if you dont understand how a piston valve works then this is a helpful link.

Step 1: Getting Parts

Well go to some hardware store and aquire all thse parts

1x 1/4in ball valve(metal)
2x 3/4in to 1/2in bushings(metal)
2x 3/4in female adapter(pvc)
1x length of 3/4in pipe of the desired size(pvc)
1x 1/2in copper cap
1x 6mm brass or aluminum tube of the desired length(might not be avaiable at your hardware store, try a hobby shop)
2x 1/2in to 1/4in bushing(metal)
1x 1/4in NPT schrader valve
3x 1/4in close nipples(prefferabley brass over steel)
1x double syringe container of epoxy
whatever tape strikes your fancy
bike inner tube

And some tools that you most likely have but if you dont you need to get them.

Scissors
Clamp
Adjustable wrench
Pipe wrench
A vice is helpful but not nesessary
A nail(or similar)

And that is it
If you dont have some of tools the project will be a lot more pricey.

Step 2: Putting Parts Together

Time to thread some of the parts together.
Make sure to add atleast 4 layers of Teflon tape to each joint before threading it in.
Take your 3/4in female adaptor(pvc) and thread your 3/4in to 1/2in bushing into it. Make sure not to overtighten here, you don't want to crack the pvc.
Next take your 1/2in to 1/4in bushing and thread it into the 3/4in to 1/2in bushing. Tighten this one as hard as you can, you don't need to worry about cracking the metal that much.
And then repeat what you did with your first female adaptor with the second female adaptor. Then with the second female adaptor thread a 1/4in nipple into the 1/2in to 1/4in bushing. Then thread a 1/4in tee to that. Thread a 1/4in nipple to one side of the tee and a schrader valve to the other side. Then thread the 1/4in ball valve to the 1/4in nipple.

And thats it for now

Step 3: The Barrel

Well you are going to need to epoxy the barrel to a 1/4in brass nipple in orded to attatch it to the gun.
Video of how to do that right here.

http://www.youtube.com/watch?v=7CUGqOPiQSw
I recommend you click on high quality at the bottom.

Next I would reccomend you clamp it in a vice for an hour or so but if you don have a vice then you are going to have to just hold it in your hand steady for about 20min and then set it down for another hour or so.
After that hour wait for it to partially dry you can screw it into the 1/2in to 1/4in bushing on that 3/4in female adaptor you put aside. Remember to teflon tape it.

Step 4: Finding the Right Size Chamber and Cutting It.

Well since your barrel is probably a different length then mine you are going to need to find a good chamber length for your barrel length.
Compare the length of your barrel to your chamber and then about 1in farther down make a mark with a pencil or pen. This is where you will cut with a hacksaw.
Next you cut at the mark you made trying to keep it as sqaure as possible. If you want you could you a mitre box but I dont assume that to be nessesary.
After you have made the cut smoothen it with course sandpaper.
And you have your chamber.

Step 5: Piston

The piston is rather simple to make.
Just take your copper cap and epoxy some inner tube cut to size onto the top. Use the same epoxying technique as when you epoxied on the barrel.
Then wrap the cap in your choice of tape until it fits well but slides easy in the 3/4in pipe.
Thats it, you piston is done.

Of course you don't need to use a copper cap for the piston. There are much better piston materials out there, such as a casted epoxy piston.
Its just the copper cap is very easy and reliable(I've shot it more 300 times without a single problem).

And sorry I have no pics.

Step 6: Cementing the Pvc Together

Well now its time for you to cement the pvc together
If youy have never solvent welded before I recommend you seek advice from a plumber. But if you dont feel like doing that then you can always just read this.

Prime one of the 3/4in female adaptors and prime the end of the pvc pipe. The add some cement on both surfaces and push it together. Hold it together for 30sec. Then do the same to the other female adaptor and end of the pipe.
Leave it for 24hrs to dry
Once it done drying your gun is complete.

Step 7: Have Fun With It

Once you are done waiting for the glue and epoxy to dry all you need to do is unthread the 3/4in to 1/2in adaptor out of the female adaptor and pop in your piston. Rethread the 3/4in to 1/2in bushing and your set.
Charge it up to whatever pressure you like(under 140psi)
And unleash by opening the ball valve.
http://www.youtube.com/watch?v=19ROhC86BmA

Remember all the safety rules stated at the beginning.